In recent years, techniques for converting natural energy, such as sunlight and wind power, into electrical energy have been receiving attention. There has been increasing demand for nonaqueous electrolyte secondary batteries as high-energy-density batteries capable of storing a large amount of electrical energy. Among nonaqueous electrolyte secondary batteries, lithium-ion secondary batteries have the advantage of being light in weight and having high electromotive forces. Lithium-ion secondary batteries, however, have disadvantages that the heat resistance is low and electrolytes are easily decomposed on surfaces of electrodes (or particle surfaces of electrode active materials) because of the use of organic electrolytic solutions containing organic solvents. The price of lithium resources is rising in association with the expansion of the market for nonaqueous electrolyte secondary batteries.
There have been advances in the development of molten salt batteries including flame-retardant molten salts serving as electrolytes. Molten salts have excellent thermal stability, relatively easily ensure safety, and are also suited for continuous use at high temperatures. A molten salt battery can include a molten salt which contains cations of an inexpensive alkali metal (in particular, sodium) other than lithium and which is used as an electrolyte, so that the production cost is low.
Although electrolytes containing molten salts (molten salt electrolytes) have higher thermal stability than that of organic electrolytic solutions, many of them are solid. Thus, the operating temperatures of batteries are liable to be high. For example, a salt (NaFSA) of a sodium ion and a bis(fluorosulfonyl)amide anion (FSA−: bis(fluorosulfonyl)amide anion), which is used as a molten salt electrolyte in a sodium molten salt battery, has a melting point of 106° C. Thus, in the case where NaFSA is used as a molten salt electrolyte alone, the operating temperature of a battery needs to be a temperature equal to or higher than the melting point of NaFSA.
To reduce the operating temperature of a molten salt battery, it has been reported that a salt (KFSA) of a potassium ion and FSA− is added to NaFSA and that an organic cation, for example, a 1-ethyl-3-methylimidazolium cation (EMI+) or a 1-ethyl-1-methylpyrrolidinium cation, is added to NaFSA (Patent Literature 1).